The present invention is directed to the structure and mode of operation of a fixture for assembling parts of an instrument such as a Wien filter with an exceptionally high degree of dimensional precision and reliability.
Various instruments are known which rely on emission of charged particles from a sample to derive characteristics of the sample. Examples of such instruments are electron microscopes (e.g., scanning electron microscopes), focused ion beam microscopes, and mass spectrometers which utilize various well known means to analyze the charged particles emitted from the sample.
These instruments generally comprise a supporting structure to which active elements such as electrostatic or magnetic pole pieces are adapted to be mounted and secured with a high degree of positional and dimensional precision since the location of those pole pieces controls the detailed nature of the electrostatic or magnetic field that they produce, and because the precision of the instrument is closely dependent on the precision of that electrostatic or magnetic field.
For facilitating the description of the present invention, it will be explained in connection with a scanning electron microscope (xe2x80x9cSEMxe2x80x9d). However, it should be understood that the invention is not limited to an SEM and can be applied by one with ordinary skill in the art to other instruments such as those mentioned above.
An SEM operates by generating a primary scanning electron beam that impacts a sample a surface of which is being imaged. As a result, backscattered and secondary electrons are emitted from the sample surface and have respective trajectories backward along the original beam direction which is perpendicular to the sample surface (known as the on-axis direction) and at angles diverging therefrom. Emitted electrons are collected by a detector, which is arranged above the sample. The detector generates a signal from the electron emission collected from the sample surface as it is exposed to the electron beam. The signal from the detector is typically processed to create an image of the surface, which is then displayed on a video screen.
With structures such as high aspect-ratio trenches and contact holes, the only electrons able to escape are those emitted on-axis. However, standard SEMs do not readily detect on-axis emitted electrons. This is because the detector must be spaced from the axis so as not to impede the primary, or incident, beam. In that position, on-axis electrons do not impinge upon the detector. Also, the high voltage on the front surface of the detector, which is commonly used to attract the secondary electrons, may adversely affect the primary beam.
The term xe2x80x9cfilterxe2x80x9d as used.herein refers to devices used to in some way separate, or disperse, the particles of a charged particle beam through their inherent differences in either velocity (although often, in fact usually, this quantity is classified as xe2x80x9cenergyxe2x80x9d), charge, or mass. This can be done through application of either electric or magnetic fields, or a combination of both.
Wien filters have long been known (see W. Wien, Ann. Phys. 65 (1898), page 444). In such a filter, electrodes and magnetic poles are simultaneously utilized to create both an electric field and a magnetic field. The two fields are tuned, or adjusted, to apply equal and opposite forces to electrons in the incident beam, so that it is not deflected. However, an electron moving in the direction opposite to the incident beam will be oppositely affected by that same magnetic field force, which acts on such electron in the same direction as the electric field force. Thus, that electron will be deflected away from the beam axis. In that way, even the on-axis particles can be detected as they are deflected to a properly positioned detector.
U.S. Pat. No. 4,658,136 of Apr. 14, 1987, entitled xe2x80x9cSecondary Electronic Detecting Apparatusxe2x80x9d suggests the use of a Wien filter in a scanning electron microscope, but in practice Wien filters have not been used in that application because they have caused relatively small but significant disturbances in the electrostatic and magnetic fields through which the particles of the primary beam and the emitted particles pass, thereby disturbing the trajectories of such particles, which disturbances have been sufficient to undesirably degrade the primary beam and hence the resolution of the SEM. More specifically, the use of a Wien filter in an SEM requires that the magnetic and electrostatic fields be precisely matched and uniform in order to ensure that the primary electron beam is unperturbed and aberrations are kept to a minimum.
In a co-pending patent application entitled xe2x80x9cWien Filter For Use In A Scanning Electron Microscope Or The Likexe2x80x9d, Ser. No. 10/010,321 filed on Nov. 8, 2001, and assigned to the assignee of this application, and hereby incorporated herein by reference, a particularly improved Wien filter structure is disclosed and claimed. It comprises, broadly considered, a ceramic supporting structure through which a plurality of magnetic pole pieces extend, the pole pieces engaging that supporting structure and being brazed thereto. It is essential to the precision of the functioning of the filter that the location of the pole pieces be controlled to a very high degree of accuracy. That accuracy may be achieved in part by precise machining of the pole pieces and supporting structure, particularly those surfaces of those elements which are pressed into engagement with one another. However, it has been found that that degree of mechanical precision is often incapable of producing sufficient accuracy of operation, particularly when the pole pieces are secured to the supporting structure by brazing, which involves subjecting the parts to very high temperatures over an extended period of time. Inaccuracies in positioning of the pole pieces often tend to occur because of temperature- or heat-related causes.
One object of the present invention is to provide a fixture for use in assembling pole pieces to a supporting structure, particularly in a Wien filter or the like, which facilitates and ensures the relative positioning of the parts, particularly when they are secured together by a brazing operation, with a significantly improved degree of accuracy and reliability, thereby to produce an instrument of high accuracy and precision.
It is a further object of the present invention to provide such a fixture which supports and guides the parts to be assembled and urges them into engagement with one another during the securing process by means which is substantially not adversely affected by the securing process itself.
It is a more specific object of the present invention to provide such a fixture in which the parts to be assembled are mounted and in which those parts, during the securing operation, are urged against one another by a gravity-actuated means.
As indicated above, while the fixture of the present invention is here specifically described for use in assembling pole pieces to a supporting structure specifically in connection with a scanning electron microscope, it will be understood that the utility of the fixture under discussion is not limited to that application, but is also advantageous in connection with other instruments where a high degree of precision is called for.
The fixture in accordance with one aspect of the present invention comprises a housing into which the supporting structure and associated pole pieces of the instrument are adapted to be received and positioned in a preliminarily assembled relationship. More specifically, the fixture comprises an open-top housing into which the supporting ring of a Wien filter may be placed and supported with its magnetic pole pieces in place, those pole pieces passing through openings in the ring structure and extending radially outwardly to define sections on which energizing coils may be mounted. The supporting ring is provided with outwardly facing surfaces and the pole pieces are provided with inwardly facing pole surfaces adapted to mate with one another and, when the parts are to be connected by brazing, with a thin layer of brazing material interposed between those surfaces. During the brazing operation, which is carried out for an extensive period of time at a highly elevated temperature, all of the parts are subjected to that heat condition. It is therefore necessary that the pole pieces be pressed against the supporting ring during the securing operation. The ultimate relative position of the pole pieces in the completed instrument determines the accuracy and, in a Wien filter, the resolution of the instrument, and that in turn will depend upon the degree and constancy to which the pole pieces are individually pressed against the supporting ring.
By utilizing a series of pole-piece-pushing elements spaced around the fixture so that each acts upon a different pole piece, and by providing for each such element a pole-push-producing instrumentality which is gravity-actuated, the desired degree of accuracy and reproducibility is obtained.
Another aspect of the present invention is directed to a fixture for assembling to a supporting structure a plurality of pieces adapted to be accurately located on and secured to the supporting structure. The fixture includes a body defining a space into which the supporting structure and the pieces are adapted to be received and located. The body includes an outer structure adapted to extend beyond outer ends of the pieces. A plurality of movable elements are mounted on the outer structure for movement in a piece-pushing direction and adapted to respectively engage the outer ends of the pieces in the course of such movement. Gravity-actuated means act on the movable elements to urge them in the piece-pushing direction for engagement with the pieces to thereby push the pieces toward and into operative engagement with the supporting structure.
A further aspect of the present invention is directed to a fixture for assembling to a supporting structure a plurality of pole pieces adapted to be spaced along and secured to the supporting structure. The fixture includes a bottom wall and a second wall extending up from the bottom wall to define an open-topped interior space into which the supporting structure and the pole pieces are adapted to be received. A series of openings in the second wall are provided so that each corresponds in location to one of the pole pieces. Arms are mounted in the openings. These arms have a first part extending radially inwardly of the second wall and adapted to engage a pole piece and urge it toward the supporting structure, and a second part extending generally outwardly of the second wall. A weight-means is operably connected to the second part and is effective to urge the second part so as to move the first part against the corresponding pole piece and urge the pole piece against the supporting structure.
Still another aspect of the present invention is directed to a method for assembling an apparatus including a supporting structure to which a plurality of pieces are to brazed. A fixture is provided having a body defining a space into which the supporting structure and the pieces are adapted to be received and located. The body includes an outer structure adapted to extend beyond outer ends of the pieces. The supporting structure and pieces are preliminarily positioned relative to each other within the space so that opposed surfaces on the supporting structure and the respective pieces are in mating engagement. A plurality of movable elements are mounted on the outer structure for movement in a piece-pushing direction and adapted to respectively engage the outer ends of the pieces in the course of such movement. A gravity-actuated means acting on the movable elements urges them in the piece-pushing direction for engagement with the pieces to thereby push the pieces to attain the mating engagement with the supporting structure. Then, the supporting structure and pieces are brazed to each other.